[unreadable] The clinical trial will assess the pharmacokinetics and safety of the novel anticoagulant, fondaparinux sodium, in pediatric patients with thromboembolism. Currently available anticoagulants have significant limitations especially as it applies to the pediatric population. Thus, novel agents with improved pharmacologic properties are needed to improve the care of this increasingly recognized complication in children. Furthermore, the only available agent for long-term anticoagulation in children with heparin-induced thrombocytopenia is warfarin. Anticoagulation with warfarin in the pediatric population is problematic due to its narrow therapeutic index and the numerous drug and food interactions necessitating frequent laboratory monitoring. In addition, oral administration of warfarin (which cannot be compounded into a liquid) is difficult in young children. Thus, a novel agent for this condition is needed and fondaparinux does not cross-react with heparin antibodies. The aims of the study are to determine the proper dosing regimen (dose and interval) and safety in patients less than 18 years of age with thrombosis or heparin-induced thrombocytopenia. Another aim is to assess the utility of thromboelastography as a monitoring tool for patients on fondaparinux. This will be a pilot study which will enroll 24 patients in 3 age cohorts. Patients will receive an initial dose of 0.1 milligram/kilogram subcutaneously daily. After the first dose, fondaparinux levels will be drawn at 2, 4, 12, and 24 hours after administration. Dose adjustments will be made based on the 4 hour (peak) level, and trough levels at 12 and 24 hours will determine if daily dosing is feasible. Thromboelastography will be performed at 2 or 4 hours and 24 hours with the results correlated with the plasma activity level. Safety will be assessed by physical examination, laboratory testing, and if necessary, diagnostic imaging to determine the incidence of minor and major bleeding. Pharmacokinetic analyses as well as safety and efficacy determinations will be made which will provide valuable information on this new anticoagulant for pediatric patients. [unreadable] [unreadable]